1. Field of the Invention
This invention relates to closed circuit breathing apparatus and more particularly to such an apparatus which provides a positive pressure at the facepiece of the wearer.
2. Description of the Prior Art
Self contained breathing systems designed to be worn on the back or front of a user such as a fireman which are generally classified as either closed circuit or open circuit types. Open circuit types are those in which exhaled or expired gases are discharged to atmosphere and not rebreathed. Open circuit systems are further divided into demand and pressure demand systems. In demand systems the pressure in the facepiece or facemask in relation to the immediate environment is positive during exhalation and negative during inhalation. In pressure demand systems the facepiece pressure in relation to the immediate environment is positive during both inhalation and exhalation.
Open circuit systems are simple and provide excellent protection and comfort. However, the high rate of gas usage and subsequent weight and size of the gas container required limit practical applications of such systems to a useful life of 30 to 45 minute duration. While open circuit systems are quite satisfactory for short term work, e.g., fighting limited fires, such systems do not provide the extended useful life (3-4 hours) required for long term work in a hostile environment, e.g., mine rescue operations.
Oxygen must be conserved in order to extend the life of a self contained breathing apparatus. A person converts only about 4% of the oxygen contained in the air into carbon dioxide. A closed circuit apparatus conserves oxygen by removing carbon dioxide from the exhaled gas and replenishing the spent oxygen so that the exhaled gas after regeneration can be rebreathed.
Prior art closed circuit systems such as the systems described in U.S. Pat. Nos. 4,362,153 ("'153 patent"), 4,879,996 ("'996 patent") and 4,498,470 ("'470 patent") while permitting extended use relative to open circuit systems either do not provide positive facepiece pressure at all times, i.e. during high inhalation rates or do so only through (1) the excessive use of pressurized air or oxygen thereby limiting the useful life or (2) the recirculation of untreated exhalation air thereby subjecting the wearer to possible carbon dioxide poisoning.
The '153 patent describes a system in which oxygen from a high pressure source is continuously supplied to a diffuser at a set rate of 5-30 liters per minute ("LPM") where it is mixed with regenerated air from a breathing bag and fed to the facepiece. The wearer presets one or more reducing valves connected to the pressurized oxygen tank to provide the desired flow rate. At rest a wearer will require about 7 to 10 LPM of air. During periods of strenuous activity the wearer will require from 100 to 150 or more LPM. If the reducing valve(s) of the '153 apparatus is set for at rest or light activity conditions, then the facepiece pressure will go negative during strenuous activity conditions. This will permit leakage of the ambient air into the facepiece. Such leakage is particularly hazardous in a closed system because it can lead to the build up of toxic gases. On the other hand, if the reducing valve(s) is set to deliver sufficient oxygen to provide positive facepiece pressure during periods of peak demand the useful life of the system will be greatly reduced.
The '966 patent describes a closed circuit system in which a spring loaded gas accumulator and counterlung are used to maintain a positive facepiece pressure. However, at peak demands, these items are augmented by a valve which bypasses a portion of the exhaled gas around the CO.sub.2 scrubber so that such untreated exhaled air flows directly back to the facepiece. The patent points out that the CO.sub.2 levels during periods high activity may reach 3% which level is above the maximum (i.e., 0.5%) dictated by federal and state regulations. Furthermore, if system leakage exceeds the preset flow rate of oxygen from the pressurized tank the system of the '966 patent will lose gas and become unable to supply the wearer's demand resulting in large negative pressures at the end of the inhalation cycle. In addition to the above disadvantages, the accumulation chambers would be relatively expensive and thus not disposable after use as a practical matter. The chambers would need to be sanitized after each use.
The '470 patent describes another closed circuit system in which a spring loaded breathing bag is employed to maintain a positive pressure at the facepiece. A source of pressurized air is supplied to the inhalation section to accommodate peak demand periods and/or leakage conditions. A separate source of pressurized oxygen and an oxygen sensor serve to maintain a preset oxygen ratio in the circulating air. The '047 system is even more complicated than the '966 patent and has many of the same deficiencies.
The above and other disadvantages of the prior art closed circuit systems are overcome by the present invention.